Optional preferential voting
Optional preferential voting (OPV) is a voting system under which voters may choose to mark a preference for one candidate (effectively voting as though it were a first-past-the-post election), all candidates or any number of candidates in between. Although complete numbering is not required under the OPV system (effectively full-preferential voting), single-preference voters may be required to use a '1' rather than a tick or cross. Some jurisdictions allow ticks or crosses as the voter's intention is clear.
OPV is used in elections in New South Wales and the Northern Territory, Australia.[1][2]
In both the Tasmanian House of Assembly and the Tasmanian Legislative Council, semi-optional voting is used, with a minimum number of preferences required to be expressed; but there is no requirement to complete the entire ballot paper. All other Australian lower house elections are use full-preferential voting, where a preference for every candidate must be indicated for the vote to be counted.
In the Victorian Legislative Council, semi-optional voting is used if a voter chooses to vote below the line.
The Australian Senate voting reform of 2016 switched from full-preferential voting to semi-optional. A minimum number is specified in the instructions on the ballot paper. Since in the past a single number '1' above the line was formal, that is still a valid vote even though voters are encouraged to number six squares.
In other countries, such as Malta, Ireland, and Northern Ireland, full preferences are not required.